The present invention relates to ceramic matrix composites and more particularly to ceramic matrix composites of complex structure combining core and surface components of differing composition and properties.
Ceramic matrix composites may be generally characterized as composite bodies comprising a matrix of a ceramic material, which may be a glass, a crystalline ceramic, or a glass-ceramic, within which is disposed a reinforcing material consisting of fibrous or particulate additives having different properties than the matrix and imparting improved strength or toughness thereto. Typical of these composites are fiber-reinforced glass-ceramic composites wherein the matrix is a glass-ceramic and the reinforcement consists of refractory inorganic fibers such as silicon carbide (e.g., silicon oxycarbide) fibers. U.S. Pat. Nos. 4,485,179 and 4,554,197 describe some of these materials.
Prospective uses for fiber-reinforced ceramic matrix composites such as described in these and other prior patents and literature include use as structural elements in high temperature environments such as heat engines. The materials to be employed for such applications must exhibit particularly good strength and toughness at elevated temperature.
An important problem which has been identified in silicon-carbide-reinforced ceramic matrix composites, arising upon exposure to temperatures in the 800.degree.-1200.degree. C. range in an oxidizing environment, is that referred to as oxidation embrittlement. Instead of exhibiting the high toughness and strength of the as-made material, the oxidized material becomes brittle and subject to sudden catastrophic breakage. R. L. Stewart et al. in "Fracture of SiC Fiber/Glass-Ceramic Composites as a Function of Temperature," Fracture Mechanics of Ceramics, R. C. Bradt et al. Ed., Volume 7, pages 33-51, Plenum (New York) 1986, attribute oxidation embrittlement to oxidative deterioration of the fiber-matrix interface.
Two fundamentally different approaches to the solution of oxidation embrittlement in silicon-carbide-fiber reinforced glass ceramics have been developed, i.e., the use of fiber coatings and the use of hybrid matrix reinforcements. Fiber coating approaches include the use of vapor-deposited boron nitride coatings, as described in U.S. Pat. No. 4,642,271, and the use of sheet silicate coatings as described in U.S. Pat. No. 4,948,758. Such coatings, hereinafter referred to broadly as oxidation-resistant coatings, are intended to provide an oxidatively stable phase that preserves an effective interface between the fiber and the matrix at high temperatures.
The hybrid reinforcement approach relies on the inclusion of secondary reinforcement phases such as silicon carbide whiskers, particles, or chopped fibers, in addition to the primary reinforcement fibers, in the ceramic matrix. The aim is to provide a toughened matrix that deflects and/or blunts cracks, thereby resisting matrix microcracking at low and moderate loads. Since oxidation embrittlement proceeds rapidly only following the appearance of microcrack damage, composites with hybrid reinforcement can generally tolerate higher stress levels prior to the onset of such embrittlement. U. S. Pat. No. 4,615,987 describes hybrid glass-ceramic matrix composites containing fibers with secondary whisker additions, while U.S. Pat. No. 4,626,515 discloses the similar hybrid fiber/whisker reinforcement of glasses.
Each of the two approaches to the oxidation embrittlement problem have drawbacks. Fiber coatings can raise the oxidation embrittlement temperature, but the composites exhibit relatively low microcrack yield points and, in some instances, reduced refractoriness resulting in deformation of the part at high temperatures. A high microcrack yield point, defined as the lowest stress under transverse loading at which non-linear stress/strain behavior (caused by matrix microcracking) appears, is desirable since the microcrack yield point is generally accepted as the maximum load at which a component would be designed to be used. Part deformation at high temperatures is undesirable for applications wherein composite parts must meet close dimensional tolerances.
On the other hand, composites with hybrid reinforcement have high microcrack yield points, but generally lower ultimate failure stresses and strains. This may be due to fiber damage inflicted by the whiskers during the processing of the hybrid, or to other factors.
Attempts to improve ultimate stress/strain in hybrid composites by using combinations of coated fibers and whisker additions have yielded some improvements in properties, but have not completely solved the problem. Limited refractoriness and lower than optimum ultimate strength are still frequently observed.
In U.S. Pat. No. 4,992,318, a lamination approach to composite fabrication was proposed wherein, to protect the ultimate strength of the composite, whisker and/or chopped fiber additives were confined to specified layers during fabrication. However, this approach requires careful control over the relative elasticity of the layers in order to avoid brittle fracture behavior in the composite product.